Local bias map using line width measurements

ABSTRACT

A method and apparatus for inspecting a reticle measures line widths using an inspection tool that images the reticle and compares the image with a design database to detect errors in real time. The differences between the line widths of patterns on the reticle and the design database are stored during the inspection procedure. The difference (or “bias”) information is then processed off-line to create a map of all the local line-width deviation values (i.e., bias) of every feature on the reticle. The resultant local bias map can be used as a feedback mechanism to improve the reticle manufacturing process, as a “go/no go” criteria for the validity of the reticle, and as a standard report shipped together with the mask to the wafer fabrication facility, where it can be used as a yield-enhancing tool.

RELATED APPLICATIONS

This application a continuation of application Ser. No. 10/322,708,filed on Dec. 19, 2002 now U.S. Pat. No. 7,133,549, which is acontinuation of application Ser. No. 09/286,498, filed on Apr. 5, 1999now abandoned, the disclosures of which Applications are incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to a method for monitoring the productionof photolithographic reticles used in the manufacture of semiconductordevices, and more particularly for inspecting reticle. The invention hasparticular applicability for in-line inspection of reticles withsubmicron design features.

BACKGROUND ART

Current demands for high density and performance associated with ultralarge scale integration require submicron features, increased transistorand circuit speeds and improved reliability. Such demands requireformation of device features with high precision and uniformity, whichin turn necessitates careful process monitoring.

One important process requiring careful inspection is photolithography,wherein masks or “reticles”, are used to transfer circuitry patterns tosemiconductor wafers. Typically, the reticles are in the form of patternchrome over a transparent substrate, and a series of such reticles areemployed to project the patterns on to the wafer in a preset sequence.Each photolithographic reticle includes an intricate set of geometricpatterns corresponding to the circuit components to be integrated ontothe wafer. Each reticle in the series is used to transfer itscorresponding pattern onto a photosensitive layer (i.e., a photoresistlayer) which has been previously coated on a layer, such as apolysilicon or metal layer, formed on the silicon wafer. The transfer ofthe reticle pattern onto the photoresist layer is conventionallyperformed by an optical exposure tool such as a scanner or a stepper,which directs light or other radiation through the reticle to expose thephotoresist. The photoresist is thereafter developed to form aphotoresist mask, and the underlying polysilicon or metal layer isselectively etched in accordance with the mask to form features such aslines or gates.

It should be appreciated that any defect on the reticle, such as anextra or a missing chrome may transfer onto the fabricated wafer in arepeated manner. Thus, any defect on the reticle would drasticallyreduce the yield of the fabrication line. Therefore, it is of utmostimportance to inspect the reticles and detect any defects thereupon. Theinspection is generally performed by an optical system, such as theRT43200™ or ARIS-i™ reticle inspection systems available from AppliedMaterials of Santa Clara, Calif.

Basically, three inspection methods are available, die-to-database,die-to-die, and reflected-to-transmitted. In the mask shop, i.e., wherethe masks and reticles are produced, typically the die-to-databasemethod is used, where the inspection system is used to scan the mask andcompared the obtained image to the database used to create the mask. Anydifference between the image and the database is flagged as a suspectlocation. On the other hand, in the wafer fabrication plant, ‘thedie-to-die method is more prevalent, where the inspection system is usedto Scan the mask and compare the image obtained from one die on the maskto another die on the same mask. Any difference between both images isflagged as a suspect location. In a reflected-to-transmitted inspection,the system is used to scan the’, mask, and an image obtained fromtransmitted light is compared to image obtained from reflected. light.In either case, the resulting-output is an indication of all thelocations on the reticle suspected to have a defect thereupon. Such anoutput is generally known in the industry as a defect map.

Another aspect of semiconductor wafer fabrication is the design rule.These design rules define, e.g., the space tolerance between devices andinterconnecting lines and the width of the lines themselves, to ensurethat the devices or lines do not overlap or interact with one another inundesirable ways. The design rule limitation is referred to as thecritical dimension (“CD”), defined as the smallest width of a line orthe smallest space between two lines permitted in the fabrication of thedevice. The CD for most ultra large scale integration applications is onthe order of a fraction of a micron.

As design rules shrink and process windows (i.e., the margins for errorin processing) become smaller, measurement of reticle features isbecoming increasingly important, since even small deviations of linewidths from design dimensions may adversely affect the performance ofthe finished semiconductor device. Conventionally, a critical dimensionscanning electron microscope (CDSEM), such as the VeraSEM™, availablefrom Applied Materials of Santa Clara, Calif., is used to measure linewidths. However, due to the slow operation of CD-SEM's, only selectedlocations (generally about 25) are examined, and statistical analysis isused to determine the quality of the CD over the entire reticle orwafer. The sample sites are usually located in areas on the reticlelikely to have deviations, and are selected based on the experience ofthe user and/or statistical techniques.

As can be appreciated, the usefulness of CD-SEM inspection of reticle CDdepends, to a great extent, on the ability to predict which sites on thereticle contain variations. Moreover, the number of CD-SEM sample sitesis typically limited, since inspection time for each site isconsiderable. Thus, significant but. unobvious CD errors may gounnoticed during inspection, such as “global” errors causing CDvariations across the reticle, which indicate reticle manufacturingproblems; e.g., a greater CD deviation in the features in the center ofthe reticle than in features at the perimeter of the reticle.

Recently, Applied Materials has introduced in its RT-8000 ml series andARIS-i reticle inspection systems a new feature, called Line Width ErrorDetector (“LWED”). In addition to the normally reported defects, asexplained above, this feature allows the system to report another typeof defect, namely line width errors. Specifically, the LWED compares thefeature sizes of the reticle under inspection with feature sizes from adesign database to determine any width deviation from the data base. Anexample of a feature width difference defect that can be discovered bythe LWED is shown in FIG. 1. Any deviations discovered by the LWED arereported on the defect map as locations suspected of having defectsthereupon.

It can be appreciated from the above that the LWED somewhat bridges thepreviously distinct issues of defect detection and CI) inspection. Suchbridging may be very beneficial to the wafer fabrication process.Specifically, perhaps the biggest technology issue in advancing opticallithography to smaller design rules is the Mask Error Factor ‘MEF’a.k.a. mask error enhancement factor “MEEF”. This factor accounts forthe observation that small variations in CD on the mask can cause largevariations on the wafer at sub-wavelength resolution. Therefore, thereexists a need for a simple, fast, and cost-effective methodology forinspection of CD errors over the entire reticle, in addition forinspecting the reticle for defects.

SUMMARY OF THE INVENTION

The present invention provides a solution for the need to inspect theentire reticle for CD deviations in addition to defect inspection. Twonotable advantages of the invention are that: 1. it monitors CD over theentire reticle, and 2. it has no associated costs in terms ofthroughput, i.e., CD monitoring is done “on the fly” as the systeminspects the reticle for defects.

According to a feature of the present invention, as the reticleinspection system scans the reticle for defects, various features on thereticle are being measured and compared against the database.Subsequently, in addition to a detect map, the system output a CDdeviations map across the entire reticle. This enables fast andinexpensive method of

monitoring the “CD error budget” and early discovery of global issues inmanufacturing the reticles. Since the CD is measured using data obtainedby the conventional reticle inspection tool during inspection, theresults and throughput of the inspection process are not adverselyeffected.

According to a particular embodiment of the present invention, adatabase corresponding to the inspected reticle is provided. When thereticle is scanned, the width of each element on the scanned reticle andin the database is measured, and a file of width differences isconstructed. Each measured difference (including zero difference) in thefile includes its coordinates on the reticle. The file is then used toconstruct a width difference map. According to one implementation, thewidth differences are color coded according to their severity. Accordingto another implementation, the severity is indicated by another axis, sothat the resulting map is three dimensional.

The invention also provides a new inspection methods, termed by theinventors die-to-design rule. According to this method, no database or.scanning of another die is required. Instead, one or more design rules(CD) are pre-programmed into the system before the inspection begins.For example, 1, 1.6, and 2.25 microns can be pre-programmed: Anymeasurement on the image of the reticle is then compared to thepre-programmed design rule, rather than to the database or another die.

The pre-programmed design rule can also be used as a filter. Accordingto this embodiment, a threshold, e.g., 15%, is also pre-programmed withthe design rules. During inspection, the system measures each width andcompares it to the pre-programmed CD's. If the variation is above the15% threshold, it is assumed that the measurement is of a feature whichneed not be measured or reported (e.g., 3 micron feature) and themeasurement is discarded. Otherwise, the difference is stored in a fileand difference maps can be constructed as in the above.

Another feature of the invention is that the user can customize thedifference map. For example, the user may indicate that a map of only 1micron feature width should be constructed. Then, the file of either ofthe above described systems can be used to construct CD differences ofonly 1 micron features by using an appropriate threshold. That is,normally there are no “close cases” in reticle design, i.e., normallyreticels are not designed to have lines of close measurement, e.g., 0.9,1, and 1,1 micron. Therefore, using, e.g., a 10% threshold would besufficient to identify all the sought features.

Additional advantages of the present invention will become readilyapparent to those skilled in this art from the following detaileddescription, wherein only the preferred embodiment of the presentinvention is shown and described, simply by way of. illustration of thebest mode contemplated for carrying out the present invention. As willbe realized, the present invention is capable of other and differentembodiments, and its several details are capable of modifications invarious obvious respects, all without departing from the presentinvention. Accordingly, the drawings and description are to be regardedas illustrative in 7 nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application with color drawingswill be provided by the Patent and Trademark Office upon request andpayment of the necessary fee.

Reference is made to the attached drawings, wherein elements having thesame reference numeral designations represent like elements throughout,and wherein:

FIG. 1 illustrates a defect in the form of an undersized feature on areticle.

FIG. 2 is a top level block diagram of the system according to anembodiment of the invention.

FIG. 3 a-3 c exemplify measurements of image and database featurewidths.

FIG. 4 exemplifies the four measurement directions utilized in thepreferred embodiment.

FIG. 5 is a block diagram of the preferred embodiment of the presentinvention.

FIG. 6 is a block diagram that illustrates an embodiment of theinvention. FIG. 7 illustrates a 4 by 3 span of an environment of featurepixels used in practicing an embodiment of the present invention.

FIG. 8A-8D are local bias maps according to embodiments of the presentinvention.

FIG. 9 is a block diagram that illustrates an embodiment of theinvention.

DESCRIPTION OF THE INVENTION

The present invention is implemented at an inspection tool 200, as shownin FIG. 2, comprising an imager 210 for imaging the surface of a reticleR at high speed, typically using a line or area CCD and an illuminationsource such as a lamp or a laser. Inspection tool 200 further comprisesa processor 220, which preferably performs the analysis disclosed hereinelectronically, and a monitor 230 for displaying results of the analysesof processor 220. Processor 220 is in communication with a conventionalreticle reference design database 250 and a memory device 240.Inspection tool 200 may be a conventional reticle inspection system,such as the RT-8200, ARIS-i, and the inspection tools described in U.S.Pat. No. 5,619,429, entitled “Apparatus and Method For Inspection Of aPatterned Object By Comparison Thereof To a Reference”, and U.S. Pat.No. 5,619,588, entitled “Apparatus and Method For Comparing and AligningTwo Digital Representations of an Image” the entire disclosures of whichare hereby incorporated by reference.

According to the methodology of the present invention, a reticle isimaged and the processor 220 compares the image with a design databaseto detect defects in real time. According to the preferred embodiment,the LWED is used to detect variations in widths of features on thereticle and create a CD error map. The LWED and its operation are fullydescribed in co-pending application Ser. No. 08/933,104, attorney docket22121 PDC which is incorporated herein by reference. Specifically, thex,y edges of each of the image pixels, i.e., the CCD pixels, is dividedinto data sub-pixels, e.g., 6 or 32 data sub-pixels. Thus, each ⅙×⅙, or1/32× 1/32 data sub-pixels forms a measure unit. Then, each feature onin the image and database is measured per measure units. An exemplarymeasurement is depicted in FIGS. 3 a-3 c.

In the example of FIG. 3 a, a database measurement of 140 units wasobtained, while in FIG. 3 b a measurement of 163 units was obtained forthe image. A database over image overlay is also depicted in FIG. 3 c toexemplify the differences. It should be appreciated that depending onthe system's optics and resolution, a measure unit may correspond tovarious absolute measurement which can be determined and calibrated foreach system. Additionally, it should be appreciated that while in thepreferred embodiment each image pixel is divided into a 6×6 or morepreferably 32×32 data sub-pixels, the image pixels need not be divided,or may divided into other sizes of sub-pixels. However, the division tosub-pixels increases the resolution and when high resolution is needed,small sub-pixels should be used.

In the preferred embodiment, four measurement directions are provided:horizontal, vertical, slash, and back-slash. These are depicted in FIG.4, and are used to cover the various orientations generally provided onreticles. Of course, if technology progresses to include otherdirections on the reticles, other measurements directions may be used.Also, in the preferred embodiment, three different sets of measurementsare used for comparison with the data base: (1) a single measurementtaken between two edges; an average of three measurements, i.e., themeasurement corresponding to (1) averaged with two adjacent neighboringmeasurements; and (3) average of five measurements, i.e., themeasurement of (3) with two additional neighboring measurements. Theseare referred to herein as Chain 1, Chain 3 and Chain 5 measurements,respectively. This averaging is advantageous to reduce edge roughnessresulting from the imaging and digitizing systems.

Each differences between the measured CD of patterns on the reticle andthe design database are. stored during the inspection procedure,together with the corresponding coordinates. The difference (or “bias”)information is then used to create maps of the CD variation of theentire reticle (sometimes referred to herein as the “Local Bias Map” or“LBM”). Thus, the present invention utilizes the fact that theinspection tool measures distances between all pattern edges on thereticle to gather all the local CD deviation values (i.e., bias) ofevery feature on the reticle. The resultant map, referred to as a “localbias map”, can be used as a feedback mechanism to improve the uniformityand quality of the reticle manufacturing process, as a basis forselecting meaningful sites for further inspection at a CD-SEM, as aquality control tool to detect, locate and measure critical CDdeviations which may ruin the final printed wafer, thus providing “go/nogo” indicator for reticle delivery to the wafer fabrication facility,and as a standard report shipped together with the mask to the waferfabrication facility, where it can be used to optimally adjust the waferprinting tools to enhance yield.

The preferred embodiment of the present invention will now be describedwith reference to FIG. 5. Specifically, in block 500, a slice of thereticle is imaged (it should be appreciated that the imaging can be donefor defect inspection purposes concurrently). Then, in block 510, afeature is selected and its width measured (Chain 1), preferably andwhen possible, in the four measure directions concurrently. Thecorresponding feature is fetched from the database and its widthmeasured in block 530. In block 520 Chain 3 and Chain 5 are obtained,i.e., an averages of the feature width and its two and four neighbors,respectively.

It should be appreciated that the measurement of the feature from thedatabase can be taken only once (i.e., Chain I only), since the databaseis presumably less “noisy” than the

image. However, it is just as easy to also obtain Chain 3 and Chain 4from the database. In such a case, each image Chain should be analyzedwith respect to the corresponding database Chain.

In block 540 difference values are obtained for chains 1, 3, and 5,either using a single database value or using corresponding databasechain values. Data pertaining to each chain is then sent to the LBMbasic unit 550. The data of each difference value includes itscoordinates, direction (horizontal, vertical, slash or backslash),polarity (corresponding to glass or chrome), and the length of thefeature as measured in the database. Each LBM Basic Unit processes thedata and sends it to the analysis and storage blocks 560. The processingof the LBM Basic Unit includes, for example, filtering according topre-programmed criteria. According to one example, each difference valueis compared to a pre-programmed Delta Max value to filter out erroneousmeasured differences caused by, for example, misregistration or patterndefect. According to another example, each difference value is comparedto a pre-programmed design rule and threshold range. If the differencevalue falls outside the threshold range, it is discarded as of nointerest. This filtering is particularly useful when monitoring aparticularly sensitive CD, while ignoring all other insensitive ones.

At this point is should be noted that the LBM basic unit 550 need notwork with single values. That is, depending on the feature size, theimage sampling rate, and the buffering of the chain data, in many casesit is possible to have more than one width value for a particularfeature. Therefore, the LBM basic unit 550 may be programmed to samplethe chain data so that it obtains average of width value, rather than asingle value. This would further help reducing the noise in the scannedimage. Preferably, the LBM basic unit 550 is provided with a thresholdvalue for sampling, so that it can check whether the number ofmeasurements in a particular area justifies averaging to reduce noise.

The output of the LBM Basic Unit sent to the analysis and storage block560 is in the form of a log file having sub sections. Each subsectionstores difference data according to their characteristics, i.e.,direction, polarity, and length. Thus, the values can be analyzed anddisplayed according to particular characteristics, as desired by theuser. For example, the user may decide to ignore all verticalmeasurements, or to limit display only to a specified, e.g., 1 micron,feature length, or to designate a different feature length for eachchain. However, in the preferred embodiment, certain analysis of the mapis performed automatically by the analysis and display unit 570, so asto assist the user in interpreting the map. For example, unit 570 mayperform certain analysis to point the user to locations on the map theuser should zoom into in order to obtain important information. The unit570 can provide the user, for example, with list of areas on the mapsuspected of being degraded, so that the user may further investigatethose areas. The list may consist ol for example, areas having highvariability of bias. Other types of statistical analysis may beperformed, such as, for example, areas of high sigma, high peak-to-peakbias values, etc. Such analysis is helpful in assisting the userunderstand the information provided in the LBM.

Referring now to FIG. 6, an exemplary process flow of another embodimentaccording to the invention is provided. At step 600 in FIG. 6, reticle Ris imaged by imager 210 in a conventional manner and the image isreceived by processor 220, which processes the image as a plurality ofdata elements called here “image pixels”. An exemplary array of pixelsP1-P12, which represent a reticle feature or “pattern” 700, isillustrated in FIG. 7. As explained above, in the preferred embodimenteach of the pixels P1-P12 is subdivided into data sub-pixels, such as to32 sub-pixels on a side to form a 32×32 matrix. Pattern 700 has edgesE1-E3 and a surface S.

Next, the width of pattern 700 is “measured” by processor 220 at step610 using the LWED. Typically, reticles are made of chrome patterns,such as pattern 700, on a transparent glass surface. When imaged withtransmitted light, the glass which transmits lights is seen as white andthe chrome which blocks the light (i.e., pattern 700) is seen as black.Line width measurements are taken, as by the LWED, on both black andwhite features, and each measurement is assigned a “polarity” dependingon whether it is white or black. However, in this embodiment, thedirection measurement is done serially and a measurement direction forall patterns on the reticle must first be chosen at step 605, eitherautomatically or by the user. As in the previous embodiment, the linewidth measurement is performed independently in up to four directions inthe patterns. In other words, up to four separate measurements are made,by analyzing the data sub-pixels in four different directions andsequences, although the reticle needs to be imaged (as by a line or areaCCD) only once.

Referring to FIG. 7, line width measurement is made horizontally (in thedirection of arrow x), vertically (in the direction of arrow y), in aslash direction or in a backslash direction. The plurality of line widthmeasurement directions is necessary because different line widthinformation is gathered in each direction during the inspection. Thishappens because during manufacture (or “writing”) of the reticle,different errors occur in different directions in the written patterns.Furthermore, the line width measurement techniques employed in thisembodiment of the present invention are most effective when measuringthe distance between parallel edges; e.g., between edges E1 and E3.Thus, it is preferred to measure in diagonal directions (slash andbackslash) as well as orthogonally.

After a width measurement of pattern 700 is completed in the chosendirection, processor 220 receives pixel data from design database 250corresponding to pattern 700 (see step 615), and performs a widthmeasurement of the reference design data, which is substantiallydefectless, in the chosen direction (see step 620). Processor 220 thencompares the width measurements from steps 610 and 620 to produce a CDdifference value (see step 625), which is positive or negative dependingon whether the width of pattern 700 is larger or smaller than the designwidth. In the preferred embodiment, steps 610, 620, and 625 areperformed for chain 1, 3, and 5, as explained above with reference toFIG. 5. The CD difference value represents the deviation of pattern700's CD from the reference design data.

The CD difference value is checked for validity by processor 220 at step630 a and, if valid, is stored in memory 240 at step 630 b, along withits location on the reticle, polarity, feature size, and direction. Ifinvalid, it is discarded at step 630 c. The validity of the CDdifference value is determined by comparing it with a maximum differencevalue. The maximum difference value is determined for each measurementbased on the polarity and size of feature 700 and the direction ofmeasurement, based on predetermined criteria which indicate that the CDdifference value is too large because of line width defects or becauseof a mismatch between the image and the database that causes themeasurements of non-corresponding features to be subtracted from eachother. If multiple sets of CD difference values were created, as bychaining, each calculated CD difference value is evaluated separately atstep 630 a, and each set is stored separately at step 630 b.

The inspection procedure of steps 615-625 described above, utilizingreference design database 250, is known as a “die-to-database”inspection; i.e., an inspection comparing the reticle under inspectionwith the data used to write the reticle. However, those skilled in theart will recognize that, instead of comparing reticle R with designdatabase 250, it can readily be compared with another die on the samereticle instead, which was previously imaged by imager 210. Such aprocedure is known as a “die-to-die” inspection.

Next, at step 0.635, processor 220 determines in a conventional mannerwhether the CD difference value represents an acceptable error or adefect, as by using measurement quality grading and/or otherpredetermined criteria. If a defect is found to exist, it is mapped in aconventional manner on a defect map (i.e., a map showing the location ofdefects) at step 640, and displayed on monitor 230. Whether or not adefect is found, at step 645 the next pattern on the reticle isselected, then measured in the chosen direction, and compared with areference pattern by repeating steps 610 to 645.

After all patterns on the reticle have been measured in the chosendirection, processor 220 generates a “local bias map” (LBM), describedin greater detail below, by plotting all the CD deviations according totheir location on the reticle, and displays the LBM on monitor 230 (seestep 650). Of course, if multiple sets of CD difference values werecreated, as by chaining, each set can be used to produce a separate LBM.Moreover, different maps can be created separately for differentabsolute CD as designated by the user.

At step 655, it is determined whether width measurements are to be takenin another direction; e.g., if a difference in bias (i.e., CD deviation)is expected between the four different directions because of theproperties of the writing tool used to produce the reticle,characterizing this difference may assist the mask shop in improving thewriting tool calibration and performance. If width measurements are notto be taken in another direction, the inspection ends at step 260.Otherwise, another direction is chosen at step 605, and the measurement,comparison, and mapping process of steps 610 to 650 is repeated.

The LBM provides information not available from the previously generateddefect map of step 640. The sole purpose of the defect map is toindicate the location on the reticle suspected of having a defectthereupon. As is well known in the art, such a defect can be a missingor extra pattern, a pin hole, a particle, etc. On the other hand, theLBM provides the user an immediate access to discover problemsconcerning the CD accuracy. The LBM plots the CD deviation at everypoint on the reticle, providing the user with a picture of the CDvariation across the entire reticle. Other values, such as the averagevalue and standard deviation of CDs across the entire reticle or inspecific parts or patterns on the reticle can also be easily calculatedand plotted using the data obtained from the LBM Basic Unit. Thus,identification of the regions of the reticle having the largest andsmallest CD variation are easily obtained. Furthermore, large scaleglobal phenomena such as gradual radial or linear change in CDvariations across the reticle, which may indicate reticle processingproblems, can be detected and measured, and local variations in bias aremapped. Alternatively, only variations that exceed pre-determinedthresholds can be mapped.

Moreover, the additional valuable information provided by the LBM, whichinformation is not provided by conventional inspection methods, isobtained using data gathered by the inspection tool during the regularreticle inspection. Thus, the present invention generates the LBM “onthe fly” without adversely affecting the regular inspection and withoutan increase in inspection time.

Regarding the LBMs, in one embodiment of the present invention, the LBMis created by assigning each difference into one of a plurality ofgroups based on the magnitude of the CD difference. Each such group hasa particular color associated therewith. Thus, each CD difference isdisplayed as a data point on a two-dimensional graph in the color of itsassigned group, wherein coordinates of the graph represent the locationon the reticle. An example of such an LBM is shown in FIG. 8A, whereinthe darkest gray regions 501 represent areas in which the local CDdeviations are the greatest. For example, the darkest gray 501correlates to about 100 nm CD deviations. The other gray zones 502, 503,504 represent areas in which a CD deviation exists, but in smallermagnitudes. The gray decreases in darkness down to the white area 505,which has virtually no CD deviations.

Another example of such a two-dimensional LBM is shown in FIGS. 8B and8C, wherein red indicates high bias (i.e., CD difference values withhigh magnitudes), blue indicates low bias, green indicates the averagebias measured across the reticle, and gray indicates areas where nomeasurements were available, due to lack of data or features that aretoo small or not selected. The color bar to the right of each map servesas a legend, indicating the magnitude differences, in nanometers,represented by the colors. FIG. 8B is the result of CD measurementstaken in the horizontal direction (in the direction of arrow x in FIG.4), and FIG. 8C is the result of CD measurements in the verticaldirection (in the direction of arrow y in FIG. 7). Differences betweenthe two maps, readily seen in the upper part of the maps, are due tothin, elongated features measurable in one direction only. An effectevident from both maps is a global CD variation that causes the bias ofthe features in the center of the reticle to be somewhat higher than thebias of features at the perimeter of the reticle.

In another embodiment of the present invention, illustrated in FIG. 8D,the LBM displays each CD difference to be mapped as a data point on athree-dimensional graph having an x-axis, a y-axis and a z-axis, whereinthe x and y axes of the graph represent the location on the baseassociated with target and reference data elements from which eachrespective CD difference to be mapped was calculated, and the z-axisrepresents a numerical value of the difference.

Since the present invention enables CD variations to be loggedindependent of the sensitivity of the regular inspection process, theuser can monitor CD variations; e.g., line-width variations, which arevery small but above an allowed tolerance, yet cannot be detected withconventional inspection methods. The monitoring of such regional CDerrors can be facilitated by magnifying or “zooming in” on a particularpart of the LBM, if desired. As noted above, the areas selected forzooming can be indicated by the unit 570 according to variousstatistical analysis of the bias data.

In an alternative embodiment of the invention, at step 250, the user canrequest that an LBM be generated showing differences (i.e., line-widthvariations) related only to features having a particular design width.Typically, reticles contain patterns, such as lines, having severaldifferent design widths; for example, 1 μm, 2 μm and 5 μm. However, theuser may be interested in monitoring the CD variation of lines of onlyone of these widths. For example, the writing tool used to produce thereticle is usually optimized to a single line width; e.g., 1 μm. Tocontrol the writing process quality, the user can obtain an LBM file for1 μm lines separately from lines of other width, allowing the mask shopto check the writing tool calibration and to check the deviation of thetool away from the 1 gm design width. Furthermore, some lines are moresensitive to CD errors than other lines, and the acceptable error offeatures of different widths may be different.

By generating an LBM of CD differences of features of only a singledesign width, the user can quickly monitor the quality of these featuresand take early corrective action and/or reject a reticle exhibitingoverly large variations in these features. Additionally, this embodimentof the present invention enables the user to observe global errorsindicating writing or processing problems, such as a difference in errorconcentration from the center to the outside of the reticle, for severaldifferent design widths separately. To facilitate generating singleline-width LBMs, the differences for features of different design widthscan be stored in separate storage devices at step 630, or in separateparts of memory 640. Moreover, each particular design rule can beassigned to a particular chain, depending on the control required.

In a further embodiment of the present invention, after generation ofthe LBM at step 650, the sum of the largest negative magnitude andlargest positive magnitude of all the CD difference values is calculatedto produce a “peak-to-peak” difference value. This peak-to-peakdifference is then compared to a predetermined threshold peak-to-peakdifference value, and the reticle is disqualified (i.e., rejected) ifthe calculated peak-to-peak difference value is greater than thethreshold value. Thus, the present invention enables a “go-no go” testfor the reticle that is superior to prior art test methodology of thistype. Prior art tests typically employ statistical methods such asmeasuring a number of points (e.g., 25 points) on the reticle with aCD-SEM and performing a peak-to-peak analysis based on those points. Thepeak-to-peak analysis of this embodiment of the present invention ismore accurate because it uses information gathered from every pattern onthe reticle, not from a sampling of a relatively small number of pointson the reticle.

According to yet another embodiment of the invention, a noveldie-to-design rule inspection method is provided. This method can beused in addition to, or instead of the die-to-database and die-to-diemethods. Specifically, according to this embodiment, one or more“absolute” CD data are programmed and stored as the reference designrule values. These may or may not be entered together with a thresholdrange, for example 1 and 1.6 microns CD value with a 10% thresholdrange. According to the die-to-design rule method, rather than comparingeach measure feature to a database or another die, it is compared to theabsolute design rule stored in the system. Any differences between themeasured feature and the programmed design rule are noted as bias. Aswith the above embodiments, the comparison can be done for Chain 1,Chain 3 and Chain 5, each against the programmed design rule. Moreover,when several design rules are used, each Chain may be assigned to handlea particular design rule.

According to further embodiment, the threshold is used as a filter.Specifically, each measurement is compared against the range provided bythe threshold. If the measurement is outside the range, e.g., below 0.9or above 1.1 microns for the 1 micron value and below 1.44 or above 1.76for the 1.6 micron value for 10% threshold, it is discarded as of nointerest. Otherwise, the system may follow one of two options: 1.calculate a difference between the measured CD and the measured CD ofthe corresponding feature in the database, or 2. calculate a differencebetween the measure CD and the absolute value. Either calculateddifference can be used to create the LBM as explained above.

FIG. 9 is a block diagram that illustrates an embodiment of theinvention shown in FIG. 2. According to this embodiment, processor 220,as shown in FIG. 2, includes a bus 902 or other communication mechanismfor communicating information, and a central processing unit (CPU) 904coupled with bus 902 for processing information. Processor 220 alsoincludes a main memory 906, such as a random access memory (RAM) orother dynamic storage device, coupled to bus 902 for storing informationand instructions to be executed by CPU 904. Main memory 906 also may beused for storing temporary variables or other intermediate informationduring execution of instructions to be executed by CPU 904. Processor220 further includes a read only memory (ROM) 908 or other staticstorage device coupled to bus 902 for storing static information andinstructions for CPU 904. A storage device 910, such as a magnetic diskor optical disk, is coupled to bus 902 for storing information andinstructions.

Storage device 910 may also serve as memory 240 in FIG. 2.

Processor 220 is coupled, as via bus 902, to monitor 230 (FIG. 2), suchas a cathode ray tube (CRT), for displaying information to the user. Aninput device 914, including alphanumeric and other keys, is coupled tobus 902 for communicating information and command selections to CPU 904.Another type of user input device is cursor control 916, such as amouse, a trackball, or cursor direction keys for communicating directioninformation and command selections to CPU 904 and for controlling cursormovement on monitor 930.

Imager 210 (FIG. 2) inputs data representative of images of a reticleunder inspection, as discussed above, to bus 902. Such data may bestored in main memory 906 and/or storage device 240, and used by CPU 904as it executes instructions. Imager 210 may also receive instructionsvia bus 902 from CPU 904. Likewise, database 250 (FIG. 2) inputs datarepresentative of a substantially defectless reticle, as discussedabove, to bus 902. Such data may be stored in main memory 906 and/orstorage device 240, and used by CPU 904 as it executes instructions.

The invention is related to the use of processor 220 for inspecting thesurface of a reticle. According to an embodiment of the invention,inspection of the reticle is provided by processor 220 in response toCPU 904 executing one or more sequences of one or more instructionscontained in main memory 906. Such instructions may be read into mainmemory 906 from another computer-readable medium, such as storage device910. Execution of the sequences of instructions contained in main memory906 causes CPU 904 to perform the process steps described above. One ormore processors in a multi-processing arrangement may also be employedto execute the sequences of instructions contained in main memory 906.In alternative embodiments, hard-wired circuitry may be used in place ofor in combination with software instructions to implement the invention.Thus, embodiments of the invention are not limited to any specificcombination of hardware circuitry and software. The programming of theapparatus is readily accomplished by one of ordinary skill in the artprovided with the flow chart of FIG. 6.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to CPU 904 for execution.Such a medium may take many forms, including but not limited to,non-volatile media, volatile media, and transmission media. Non-volatilemedia include, for example, optical or magnetic disks, such as storagedevice 910. Volatile media include dynamic memory, such as main memory906. Transmission media include coaxial cable, copper wire and fiberoptics, including the wires that comprise bus 902. Transmission mediacan also take the form of acoustic or light waves, such as thosegenerated during radio frequency (RF) and infrared (IR) datacommunications. Common forms of computer-readable media include, forexample, a floppy disk, a flexible disk, hard disk, magnetic tape, anyother magnetic medium, a CD-ROM, DVD, any other optical medium, punchcards, paper tape, any other physical medium with patterns of holes, aRAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip orcartridge, or any other medium from which a computer can read.

Various forms of computer-readable media may be involved in carrying outone or more sequences of one or more instructions to CPU 904 forexecution. For example, the instructions may initially be borne on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to processor 220 can receivethe data on the telephone line and use an infrared transmitter toconvert the data to an infrared signal. An infrared detector coupled tobus 902 can receive the data carried in the infrared signal and placethe data on bus 902. Bus 902 carries the data to main memory 906, fromwhich CPU 904 retrieves and executes the instructions. The instructionsreceived by main memory 906 may optionally be stored on storage device910 either before or after execution by CPU 904.

Thus, the methodology of the present invention enables CD variations tobe quickly mapped for analysis, without affecting the regular inspectionprocedure. By analyzing the LBMs of a reticle, the user of the presentinvention can monitor CD error patterns (e.g., global errors and errorsof a particular feature size) in ways that are not possible withconventional inspection and/or methodology tools. Furthermore, thegeneration of the LBMs of the present invention is much faster thanmeasurements using a CD-SEM, and the CD variation information yielded bythe LBMs is more comprehensive than that provided by prior art CD-SEMs,since the LBMs plot CD variation for the entire reticle, not only samplesites. Such information can be used to diagnose and correct reticleproduction problems. Moreover, since the present invention utilizesinformation gathered by the reticle inspection tool as it scans theentire reticle for defects during the regular inspection procedure, theCD deviation maps are, in essence, provided without cost to the user interms of time or effort.

The LBMs may also be used to select sites for further detailedinspection by a CD-SEM in a more meaningful way than prior art siteselection techniques, since the LBMs provide information relating toevery site on the reticle. The LBMs can be viewed, in this regard, as agood “first cut” for CD-SEM inspection site selection, and since theLBMs are made on the fly, there is no time penalty involved.Furthermore, the user can monitor errors that are too small to bepracticably detected with conventional inspection methodology, yet arelarge enough to adversely affect the performance of semiconductordevices produced using the reticle under inspection.

Moreover, the LBMs can be shipped along with the reticle to the user ofthe reticle; i.e., the wafer fabrication shop, to provide the reticleuser with information as to how the reticle should be used to obtain thebest results. The fabrication shop can then adjust its processparameters accordingly. For example, if the LBMs for a reticle show aglobal error of a particular average amount, the fabrication shop canadjust its stepper to compensate for the error, thereby increasing yieldand reducing manufacturing costs. The reticle user can also employ theLBMs to determine the extent to which the reticle consumes the user's CDerror “budget”; i.e., the amount of allowable CD variation in thefinished semiconductor devices. By allowing the user to observe globalreticle problems causing CD variation, early corrective action can betaken, thereby enabling the manufacture of devices closer to designspecifications. Still further, this LBM data can be fed back to the maskproduction shop and used to improve successive masks.

The present invention is applicable to the inspection of anyphotolithographic reticle used in the manufacture of semiconductordevices, and is especially useful for in-process inspection of reticlesused in the manufacture of high density semiconductor devices withsubmicron design features.

The present invention can be practiced by employing conventionalmaterials, methodology and equipment. Accordingly, the details of suchmaterials, equipment and methodology are not set forth herein in detail.In the previous descriptions, numerous specific details are set forth,such as specific materials,

structures, chemicals, processes, etc., in order to provide a thoroughunderstanding of the present invention. However, it should be recognizedthat the present invention can be practiced without resorting to thedetails specifically set forth. In other instances, well knownprocessing structures have not been described in detail, in order not tounnecessarily obscure the present invention.

Only the preferred embodiment of the present invention and but a fewexamples of its versatility are shown and described in the presentdisclosure. It is to be understood that the present invention is capableof use in various other combinations and environments and is capable ofchanges or modifications within the scope of the inventive concept asexpressed herein.

1. A method of inspecting target patterns formed on a surface of areticle, which method comprises: imaging the target patterns to producea plurality of target data elements representative of the targetpatterns, each target data element associated with a respective locationon the surface; receiving a plurality of reference data elementscorresponding to substantially the same respective location on thesurface as the target data elements; comparing the target data elementswith the reference data elements to obtain differences between thetarget data elements and the corresponding reference data elements;calculating the sum of the largest negative magnitude of all thedifference values and largest positive magnitude of all the differencevalues to produce a peak-to-peak difference value; comparing thepeak-to-peak difference value with a threshold peak-to-peak differencevalue; and generating a graphical display map of substantially all thedifferences as a function of the locations on the surface associatedwith the target and the reference data elements from which eachrespective difference was obtained.
 2. The method of claim 1, whereinthe patterns include pattern surfaces and edges, each target andreference data element represents at least one of a pattern surface andan edge of one of the patterns, and the comparing step comprisesprocessing the target and reference data elements in a mannercorresponding to a predetermined direction in the patterns to measuretarget and reference pattern widths between two edges; wherein thedifferences are differences between the corresponding target andreference pattern widths.
 3. The method of claim 1, further comprisingstoring all of the differences prior to mapping.
 4. The method of claim2, wherein the target patterns and the corresponding reference patternseach comprise a first plurality of patterns having a first design widthand a second plurality of patterns having at least some design widthsdifferent from the first design width, and the mapping step comprisesmapping only the differences between the first pluralities of target andreference patterns.
 5. The method of claim 4, comprising storing thedifferences between the first pluralities of target and referencepatterns separately from the differences between the second pluralitiesof target and reference patterns prior to mapping.
 6. The method ofclaim 1, wherein the mapping step comprises: assigning each differenceto be mapped into one of a plurality of groups based on a numericalvalue of the difference; associating a different color with each of theplurality of groups; and displaying each difference to be mapped as adata point on a two-dimensional graph in the color of its assignedgroup, wherein coordinates of the graph represent the location on thesurface associated with target and reference data elements from whicheach respective difference to be mapped was calculated.
 7. The method ofclaim 1, wherein the mapping step comprises displaying each differenceto be mapped as a data point on a three-dimensional graph having anx-axis, a y-axis and a z-axis, wherein the x and y axes of the graphrepresent the location on the surface associated with target andreference data elements from which each respective difference to bemapped was calculated, and the z-axis represents a numerical value ofthe difference.
 8. The method of claim 3, wherein the surface is areticle used in manufacturing semiconductor devices, and wherein each ofthe differences is expressed as a positive value or a negative value,the method further comprising: disqualifying the reticle if thepeak-to-peak difference value is greater than the threshold peak-to-peakdifference value.
 9. The method of claim 2, wherein the predetermineddirection in the patterns comprises a horizontal direction, verticaldirection, slash direction or backslash direction.
 10. The method ofclaim 1, wherein the reference data elements are received from asubstantially defectless reference design database.
 11. The method ofclaim 2, comprising: measuring a plurality of the target pattern widths;calculating an average of the plurality of target pattern widths; andusing the average as the target data element in the comparing step. 12.The method of claim 2, comprising comparing the differences to apredetermined maximum difference value, wherein the mapping stepcomprises mapping only the differences less than or equal to the maximumdifference value.
 13. A computer-readable medium bearing instructionsfor inspecting target patterns formed on a surface of a reticle, saidinstructions, when executed, being arranged to cause one or moreprocessors to perform the steps of: controlling an imager to image thetarget patterns to produce a plurality of target data elementsrepresentative of the target patterns, each target data elementassociated with a respective location on the surface; receiving aplurality of reference data elements corresponding to substantially thesame respective location on the surface as the target data elements;comparing the target data elements with the reference data elements toobtain differences between the target data elements and thecorresponding reference data elements; calculating the sum of thelargest negative magnitude of all the difference values and largestpositive magnitude of all the difference values to produce apeak-to-peak difference value; comparing the peak-to-peak differencevalue with a threshold peak-to-peak difference value; and generating agraphical display map of substantially all the differences as a functionof the locations on the surface associated with the target and thereference data elements from which each respective difference wasobtained.
 14. The computer-readable medium according to claim 13,wherein the patterns include pattern surfaces and edges, and each targetand reference data element represents at least one of a pattern surfaceand an edge of one of the patterns; wherein the instructions, whenexecuted, are arranged to cause the one or more processors to comparethe target and reference data elements by processing the target andreference data elements in a manner corresponding to a predetermineddirection in the patterns to measure target and reference pattern widthsbetween two edges; and wherein the differences are differences betweenthe corresponding target and reference pattern widths.
 15. Thecomputer-readable medium according to claim 14, wherein theinstructions, when executed, are arranged to cause the one or moreprocessors to store all of the differences prior to mapping.
 16. Thecomputer-readable medium according to claim 14, wherein the targetpatterns and the corresponding reference patterns each comprise a firstplurality of patterns having a first design width and a second pluralityof patterns having at least some design widths different from the firstdesign width; and wherein the instructions, when executed, are arrangedto cause the one or more processors to map only the differences betweenthe first pluralities of target and reference patterns.
 17. Thecomputer-readable medium according to claim 16, wherein theinstructions, when executed, are arranged to cause the one or moreprocessors to store the differences between the first pluralities oftarget and reference patterns separately from the differences betweenthe second pluralities of target and reference patterns prior tomapping.
 18. The computer-readable medium according to claim 13, whereinthe instructions, when executed, are arranged to cause the one or moreprocessors to map the differences by: assigning each difference to bemapped into one of a plurality of groups based on a numerical value ofthe difference; associating a different color with each of the pluralityof groups; and displaying each difference to be mapped as a data pointon a two-dimensional graph in the color of its assigned group, whereincoordinates of the graph represent the location on the surfaceassociated with target and reference data elements from which eachrespective difference to be mapped was calculated.
 19. Thecomputer-readable medium according to claim 13, wherein theinstructions, when executed, are arranged to cause the one or moreprocessors to map the differences by displaying each difference to bemapped as a data point on a three-dimensional graph having an x-axis, ay-axis and a z-axis, wherein the x and y axes of the graph represent thelocation on the surface associated with target and reference dataelements from which each respective difference to be mapped wascalculated, and the z-axis represents a numerical value of thedifference.
 20. The computer-readable medium according to claim 14,wherein the instructions, when executed, are arranged to cause the oneor more processors to perform the step of: disqualifying the reticle ifthe peak-to-peak difference value is greater than the thresholdpeak-to-peak difference value.
 21. The computer-readable mediumaccording to claim 14, wherein the predetermined direction in thepatterns comprises a horizontal direction, vertical direction, slashdirection or backslash direction.
 22. The computer-readable mediumaccording to claim 13, wherein the instructions, when executed, arearranged to cause the one or more processors to receive the referencedata elements from a substantially defectless reference design database.23. The computer-readable medium according to claim 14, wherein theinstructions, when executed, are arranged to cause the one or moreprocessors to perform the steps of: measuring a plurality of the targetpattern widths; calculating an average of the plurality of targetpattern widths; and using the average as the target data element in thecomparing step.
 24. The computer-readable medium according to claim 14,wherein the instructions, when executed, are arranged to cause the oneor more processors to compare the differences to a predetermined maximumdifference value and to map only the differences less than or equal tothe maximum difference value.
 25. An inspection tool for inspectingtarget patterns formed on a surface of a reticle, the inspection toolcomprising: an imager for imaging the target patterns to produce aplurality of target data elements representative of the target patterns,each target data element associated with a respective location on thesurface; a processor for receiving a plurality of reference dataelements corresponding to substantially the same respective location onthe surface as the target data elements; a comparator for comparing thetarget data elements with the reference data elements to obtainsubstantially all differences between the target data elements and thecorresponding reference data elements, wherein the processor isconfigured to calculate the sum of the largest negative magnitude of allthe difference values and largest positive magnitude of all thedifference values to produce a peak-to-peak difference value; comparethe peak-to-peak difference value with a threshold peak-to-peakdifference value; and generate a graphical display map of substantiallyall the differences as a function of the locations on the surfaceassociated with the target and the reference data elements from whicheach respective difference was obtained.
 26. The inspection tool ofclaim 25, wherein the patterns include pattern surfaces and edges, andeach target and reference data element represents at least one of apattern surface and an edge of one of the patterns; the processor isfurther configured to process the target and reference data elements ina manner corresponding to a predetermined direction in the patterns tomeasure target and reference pattern widths between two edges; and thecomparator is further configured to obtain the differences betweencorresponding target and reference pattern widths.
 27. The inspectiontool of claim 25, further comprising a storage device for storing all ofthe differences.
 28. The inspection tool of claim 26, wherein the targetpatterns and the corresponding reference patterns each comprise a firstplurality of patterns having a first design width and a second pluralityof patterns having at least some design widths different from the firstdesign width, and the processor is further configured to map only thedifferences between the first pluralities of target and referencepatterns.
 29. The inspection tool of claim 28, wherein the storagedevice is further configured to store the differences between the firstpluralities of target and reference patterns separately from thedifferences between the second pluralities of target and referencepatterns.
 30. The inspection tool of claim 25, further comprising amonitor for displaying the results of mapping.
 31. The inspection toolof claim 30, wherein the processor is further configured to: assign eachdifference to be mapped into one of a plurality of groups based on anumerical value of the difference; associate a different color with eachof the plurality of groups; and cause the monitor to display eachdifference to be mapped as a data point on a two-dimensional graph inthe color of its assigned group, wherein coordinates of the graphrepresent the location on the surface associated with target andreference data elements from which each respective difference to bemapped was calculated.
 32. The inspection tool of claim 30, wherein theprocessor is further configured to cause the monitor to display eachdifference to be mapped as a data point on a three-dimensional graphhaving an x-axis, a y-axis and a z-axis, wherein the x and y axes of thegraph represent the location on the surface associated with target andreference data elements from which each respective difference to bemapped was calculated, and the z-axis represents a numerical value ofthe difference.
 33. The inspection tool of claim 26, wherein the surfaceis a reticle used in manufacturing semiconductor devices, and whereineach of the differences is expressed as a positive value or a negativevalue, wherein the processor is further configured to: disqualify thereticle if the peak-to-peak difference value is greater than thethreshold peak-to-peak difference value.
 34. The inspection tool ofclaim 25, wherein the predetermined direction in the patterns comprisesa horizontal direction, a vertical direction, a slash direction, and abackslash direction.
 35. The inspection tool of claim 25, wherein theprocessor is further configured to receive the reference data elementsfrom a substantially defectless reference design database.
 36. Theinspection tool of claim 25, wherein the processor is further configuredto: measure a plurality of the target pattern widths; and calculate anaverage of the plurality of target pattern widths; wherein thecomparator is further configured to use the average as the target dataelement in the comparing step.
 37. The inspection tool of claim 25,wherein the comparator is further configured to compare the differencesto a predetermined maximum difference value, and the processor isfurther configured to map only the differences less than or equal to themaximum difference value.